With rapid development of modern industry, there is an ever-increasing demand for power supply systems, and environment-friendly and energy saving power supplies have already become an application trend. Therefore, power conversion topologies having high efficiency and high power density have become favored in the industry. Due to their features including high efficiency and high power density, bridgeless PFC circuits are gradually applied to power supply modules having high efficiency and high power density.
In a bridgeless PFC circuit, an induction current is desired to be sampled to perform loop control so as to ensure that an AC input current follows an AC input voltage, thereby implementing the functionality of power factor correction. Compared to a traditional PFC circuit, the bridgeless PFC circuit has a boost inductor directly coupled to an AC voltage input, which increases the difficulty for sampling the induction current.
For a bridgeless PFC circuit, an existing induction current sampling method is described as follows.
FIG. 1 is a schematic diagram of bi-resistor sampling mode in the prior art. As shown in FIG. 1, in switching branches of the bridgeless PFC circuit, there are two serially-connected sampling units Rs1, Rs2 for sampling currents flowing through respective bridgeless PFC switch transistors, i.e., rise edges of induction currents in the bridgeless PFC circuit. Using such a way to sample an induction current of the bridgeless PFC circuit has the following drawbacks: when an AC input voltage is very high, the bridgeless PFC boost circuit has a very small duty cycle, and due to the existence of problems such as sampling delay, the sampling circuit cannot sample the induction current, this then results in loss of control of the control loop and thus threatens reliability of the power supply.
FIG. 2 is a schematic diagram of a bi-current-transformer sampling mode in the prior art, as shown in FIG. 2, a first current transformer P1 is connected in serial between an inductor L12 and a switch transistor S12 of the bridgeless PFC circuit; and a second current transformer P2 is connected in serial between an inductor L22 and an anode of a diode D22 of the bridgeless PFC circuit. A current flowing through the bridgeless PFC switch transistor is characterized by calculation of the sum (I1+I2) of a current flowing through the first current transformer and a current flowing through the second current transformer, i.e., rise edges of induction currents in the bridgeless PFC circuit. Similarly, this sampling device has the following drawbacks: when an AC input voltage is very high, the bridgeless PFC boost circuit has a very small duty cycle, and due to the existence of problems such as sampling delay, the sampling circuit cannot sample the induction current, this then results in loss of control of the control loop and thus threatens reliability of the power supply.